Fix -Wdeprecated-copy-dtor and -Wdeprecated-dynamic-exception-spec warnings.

Summary:
The former are like:

libcxx/include/typeinfo:322:11: warning: definition of implicit copy constructor for 'bad_cast' is deprecated because it has a user-declared destructor [-Wdeprecated-copy-dtor]
  virtual ~bad_cast() _NOEXCEPT;
          ^
libcxx/include/typeinfo:344:11: note: in implicit copy constructor for 'std::bad_cast' first required here
    throw bad_cast();
          ^

Fix these by adding an explicitly defaulted copy constructor.

The latter are like:

libcxx/include/codecvt:105:37: warning: dynamic exception specifications are deprecated [-Wdeprecated-dynamic-exception-spec]
    virtual int do_encoding() const throw();
                                    ^~~~~~~

Fix these by using the _NOEXCEPT macro instead.

Reviewers: EricWF, mclow.lists, ldionne, #libc

Reviewed By: EricWF, #libc

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D76150

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 585a3cc31bb49f2a526dcb74dc9a980379193f93
diff --git a/include/exception b/include/exception
index c7dcac2..8e32979 100644
--- a/include/exception
+++ b/include/exception
@@ -98,6 +98,8 @@
 {
 public:
     _LIBCPP_INLINE_VISIBILITY exception() _NOEXCEPT {}
+    _LIBCPP_INLINE_VISIBILITY exception(const exception&) _NOEXCEPT = default;
+
     virtual ~exception() _NOEXCEPT;
     virtual const char* what() const _NOEXCEPT;
 };